1. Field
The invention relates generally to wireless communication, and more particularly to processing signals based on the position of a remote terminal relative to a base station.
2. Description of the Related Art
A wireless communication system may comprise multiple remote terminals and multiple base stations. Communication between the remote terminals and the base stations travel over a wireless channel and can be accomplished using one of a variety of multiple access techniques which facilitate a large number of users in a limited frequency spectrum. Examples of multiple access techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
Systems based on CDMA may provide certain advantages over other types of multiple access systems. For example, CDMA permits the frequency spectrum to be reused multiple times, thereby permitting an increase in system user capacity. Additionally, use of CDMA techniques permits the special problems of the wireless channel to be overcome by mitigation of the adverse effects of multipath, e.g. fading, while also exploiting the advantages thereof. A CDMA system is typically designed to implement one or more standards, such as IS-95, CDMA2000, and WCDMA standards, all of which are known in the art and incorporated in their entirety herein.
In a system based on CDMA, communication signals are “spread” using a pseudorandom (PN) code. Because the signals share a common frequency spectrum, individual signals are distinguished by their unique PN code. In a CDMA system the communication signals are also synchronized to each other.
In a typical CDMA system the signals transmitted from base stations to remote terminals, commonly referred to as the forward link, include a pilot signal that has a common PN sequence. Each base station transmits its pilot signal with an offset in time from the pilot signals of neighboring base stations, so that the pilot signals can be distinguished from one another at the remote terminal. At any given time, the remote terminal may receive a variety of pilot signals from multiple base stations. Using a copy of the PN sequence produced by a local PN generator, the terminal can determine the relative phase, or PN offset, of the received pilot signals, and thereby identify the corresponding base station that transmitted the pilot signal. The relative phase of the pilot signals is measured by matching, or correlating, the local PN sequence to the received signals. Correlating to the pilot channel provides a coherent phase reference for demodulating other communication signals sent on the forward link. Although there is no pilot signal sent by the remote terminal to the base station, on what is commonly referred to as the reverse link, the remote terminal is assigned a unique PN sequence used to transmit signals to the base station signal. The base station correlates to the PN sequence assigned to the remote unit in the reverse link signal, and can thereby identify which of the remote terminals the signal was transmitted by.
Typically, a search engine, used in the correlation process, steps through a set of PN offsets, commonly called a search window, that is likely to contain the PN offset of the communication signal. For example, the nominal PN offset of the pilot signals received at a remote terminal are not only the result of the offsets introduced into the pilot signal by the individual base station but they are also due to the relative location of the remote terminal to the various base stations. Because the pilot signals travel different distances from various base stations to a remote terminal, the pilot signals received by the remote terminal will be delayed, and therefore offset, by different amounts of time due to the differing distance each of the individual pilot signals traveled. The uncertainty in the PN offset of the received pilot signal causes the remote terminal to search through large search windows, consuming scarce resources in the terminal that could be utilized for other functions.
Determining the nominal PN offset is even more complicated if the remote terminal is mobile and moving in relation to the base stations. In a typical mobile terminal, to conserve power and extend battery life, the terminal may enter a “sleep mode” where most communication functions, including the search engine, have power removed. If the mobile terminal moves relative to the base station before power is reapplied, the nominal PN offset of the pilot signal received from a base station will have changed. Thus, even if the nominal PN offset of a pilot signal is known when the remote unit went to “sleep,” when the mobile terminal wakes up the nominal PN offset can be different and a new search will have to be performed that will consume additional remote terminal resources.
An additional problem introduced by movement of a mobile terminal is that while the terminal is in motion relative to a base station, there is an apparent change in the frequency of signals received at both the terminal and the base station. This apparent change in frequency is due to the well known phenomenon referred to as Doppler shift. Frequency changes due to the Doppler shift require both the mobile terminal and the base station to perform searches using different frequency hypotheses, and then determine which hypothesis produces the best result. Again, searching using various hypotheses consumes resources.
There is therefore a need in the art for techniques to provide an improved estimate of the nominal PN offset of pilot signals. In addition, there is a need in the art for techniques to improve the selection of frequency hypotheses.